1. Field of the Invention
The present invention relates to a fuse circuit formed in a semiconductor integrated circuit.
2. Description of the Related Art
A semiconductor integrated circuit generally has a redundancy circuit for relieving a lattice defect in a substrate and a defect caused in a fabrication process. For example, in a semiconductor memory such as DRAM, a redundancy memory cell array is formed in addition to regular memory cell arrays. The semiconductor memory having the redundancy memory cell array includes a plurality of fuse circuits for storing respective bits of addresses indicating the memory cell arrays. Polysilicon has been used conventionally as a material of a fuse which makes up the fuse circuit, but metal such as aluminum or copper is often used in recent years.
When a memory cell array is defective, the fuse of a predetermined fuse circuit is blown (programmed) according to the address of the defective memory cell array in a test process. The fuse is blown by, for example, laser irradiation.
The fuse circuit of this kind outputs a fuse signal indicating a blown-out state of the fuse when the power of the semiconductor memory turns on. Then, the semiconductor memory disables the defective memory cell array according to the state of the fuse circuit, and enables the redundancy memory cell array instead. Namely, the defective memory cell is relieved. Thus, the defective memory cell is relieved according to the address programmed in the fuse circuit, thereby improving yield.
It should be mentioned that it is hard to blow the fuse formed of metal as compared with the fuse formed of polysilicon. For this reason, there may be the case where the fuse formed of metal is not cut off completely by a single laser irradiation. Even though the fuse is not cut off completely, it is determined that the fuse is blown when a resistance value of the fuse exceeds a predetermined value. Hence, the fuse circuit operates normally.
When the fuse is formed of metal, however, a growback phenomenon, occurs due to a voltage difference between both ends of the fuse, if the fuse is not cut off completely. The growback phenomenon means that the resistance value of the fuse decreases gradually due to electromigraion and the like.
When the resistance value of the fuse decreases to be equal to or smaller than the predetermined value due to the growback, the fuse circuit determines that the fuse is not blown and outputs the fuse signal. Hence, when the phenomenon like the above occurs, the defective memory cell array is not relieved properly and a malfunction occurs in the above-described semiconductor memory.
In order to prevent the growback, the fuse circuit, in which the fuse is formed of metal, conventionally cuts the fuse completely off by, for example, carrying out blowout processing twice. However, the semiconductor memory such as the DRAM has a larger number of the fuses as compared with a logic LSI and the like. For this reason, there is a disadvantage that performing the blowout processing twice increases processing time in the test process.
It is an object of the present invention to provide a fuse circuit whose characteristics will not change even in long-term use.
It is another object of the present invention to shorten a time for blowing out a fuse of the fuse circuit.
According to one of the aspects of the fuse circuit of the present invention, a fuse is connected between a first node and a second node. A first switch is connected between the first node and a first power supply line. A second switch is connected between the second node and a second power supply line. A load element is connected between the first node and the second power supply line. A third switch is connected between the second node and a third node. A holding circuit is connected to the third node. The holding circuit holds a level of the third node and outputs the held level as a fuse signal indicating a blown-out state of the fuse.
The first switch turns on during a first period to connect the first node to the first power supply line. The second switch turns on before and after the first period and turns off during the first period. The third switch turns on in the first half of the first period and turns off in the second half of the first period. A level of the second node is transmitted to the third node during an period of the third switch.
In the case where the fuse is not blown, the level of the second node turns to that of the first power supply line by the turning-on of the first switch. Since the third switch has turned on in the first half of the first period, the level of the second node is transmitted to the third node. The holding circuit holds the level of the first power supply line (indicating that the fuse is not blown), and outputs the held level as the fuse signal.
The third switch turns off in the second half of the first period. After completion of the first period, the levels of the first node and the second node get fixed at a level of the second power supply line by a current flowing through the load element or the turning-on of the second switch. At this point the third switch is off, and hence it is possible to prevent a short circuit between the second node (voltage of the second power supply line) and the third node (voltage of the first power supply line). This consequently prevents data (level) destruction in the holding circuit, and a malfunction of the fuse circuit.
Meanwhile, in the case where the fuse is blown, the level of the first node is not transmitted to the second node even if the first switch turns on. Hence, the level of the second node is kept at the level of the second power supply line. Since the third switch turns on in the first half of the first period, the level of the second node is transmitted to the third node. The holding circuit holds the level of the second power supply line (indicating that the fuse is blown), and outputs the held level as the fuse signal. Thereafter, the second switch turns on, whereby the level of the second node gets fixed at the level of the second power supply line. Further, the level of the first node gets fixed at the level of the second power supply line by the current flowing through the load element.
Thus, a voltage difference between both ends of the fuse (the first and second nodes) becomes 0 V after the completion of the first period (after the holding circuit""s holding the levels), regardless of whether or not the fuse is blown. Therefore, it is possible to prevent the occurrence of the growback due to electromigration and the like and to prevent gradual decrease in a resistance value of the fuse, even when the blown fuse is not completely cut off. In other words, characteristics of the fuse circuit will not be deteriorated even in long-term use. This makes it possible to prevent the fuse circuit from erroneously outputting fuse signals, and to prevent the malfunction of a semiconductor integrated circuit in which the fuse circuit is formed.
The growback does not occur even when the fuse is not cut off completely so that just one fuse blowing out is sufficient for the fuse circuit. This can consequently shorten the time for blowing out the fuses in a test process and the like.
According to another aspect of the fuse circuit of the present invention, applying the present invention to the fuse circuit having a fuse formed of metal enables secure prevention of the occurrence of the growback due to electromigration and the like even if the blown fuse is not cut off completely.
According to another aspect of the fuse circuit of the present invention, the load element is formed as a load transistor to turn on after the first period. Since the load element is formed with the transistor, the level of the first node can be changed to that of the second power supply line at desired timing after the first period. As a result of this, the voltage difference between both ends of the fuse can quickly become 0 V after the completion of the first period.
According to another aspect of the fuse circuit of the present invention, the load element is formed as a resistor. Forming the load element with the resistor allows the level of the first node to turn to the level of the second power supply line without using a control signal. As a result, a number of signal lines can be reduced.